Horizontal cracks are usually the result of the either improper fastening or if it is isolated most likely the result of a drywall board that was damaged prior to being installed. A 1/2"x4’x12 sheet weighs approximately 82 pounds…imagine dropping those sheets from about 36 inches onto their side… while you may not see any external damage you can rest assure there is internal stress cracks that often show up after its hung.
I had to go back and replace about 10 sheets on a custom home… had the installer and supplier come back out at inspect same… they both agreed it was damaged prior to installation…fortunately the supplier fitted the bill. Since that time myself or an employee will show up for deliveries.
Cracks will also occur when an uneducated installer puts butt joints over doors and windows. With the open design of so many homes you will have butt joints however knowing where to place the butt joint goes along way in alleviating potential cracks.
Nail pops are often the result of the nail or screw being driven too deep (should not break the paper) or using too long of a nail or screw. With drywall the optimum fastener size is that which is just enough to secure it… typically 1"
I know up north we use to glue and screw… I have not so much seen that here in the south. We also install fireblocking at all horizontal seams… it strengthens the walls and gives a better finish.
As to your question about report it… unless there is any other indication that it is a structural issue, I would disclose that drywall can and often does crack as a result of multiple issues… be it weather, installation or manufacturer however unless those cracks are greater than 3/16" it is not a condition that will adversely affect the home.
hope that helps.